1. Field of the Invention
The present invention relates to a semiconductor substrate and a fabrication method thereof and more particularly, to a single-crystal silicon (Si) substrate having a low oxygen-concentration layer and a fabrication method of the substrate.
2. Description of the Prior Art
In recent years, integration and miniaturization of the semiconductor integrated circuit devices (ICs) using a single-crystal Si substrate have been progressing more and more. Under such the circumstances, the need to accurately control the concentration and profile of impurity-doped regions such as well regions and source/drain regions has been becoming stronger and stronger.
On the other hand, the impurity-doped regions have been usually formed by an ion-implantation process.
To accurately control the concentration and profile of the impurity-doped regions, the dopant atoms introduced into the Si substrate by the ion-implantation process are required not to cause unwanted diffusion in subsequent fabrication processes for the ICs.
It is popular that the dopant atoms introduced by the ion-implantation process are electrically inactive. Therefore, to make the ion-implanted substrate suitable for the ICs, the dopant atoms usually need to be electrically activated by a heat-treatment or rapid annealing process.
To make it sure that the introduced dopant atoms are electrically activated, the heat-treatment or annealing process has been typically performed at a temperature as high as 800.degree. C. or higher for a short time (e.g., several seconds) or at a temperature as high as approximately 700.degree. C. for a long time (e.g., several minutes).
However, if the annealing process is performed at 800.degree. C. or higher, there is a problem that the introduced dopant atoms tend to diffuse in the substrate, resulting in unwanted change of the concentration and profile of the impurity-doped region. On the other hand, if the annealing process is performed at 700.degree. C. or lower, there is a problem that the annealing process necessitates a long processing time and as a result, this annealing process is not practical for actual IC fabrication.
Moreover, there is another problem that the substrate tends to be contaminated during the annealing process due to the long processing time.